Combine offsets in loop-based dynamic BCE.

Rationale:
Similar to what I did recently for dom-based dynamic BCE, this
CL combines offsets for the tests generated for loop-based
dynamic BCE. For a set of n references, this reduces the
number of generated tests from 2*n+1 down to at most 4
(in some cases even less).

TEST: 530-checker-loops3

BUG=27430379

Change-Id: Ic80c2563eaae23f514c1fd52965dd83bccb9d190